*
   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
   *     graph
   * @since 23.0 (since 20.0 with return type {@code V})
   */
  Optional<V> edgeValue(N nodeU, N nodeV);

  /**
   * Returns the value of the edge that connects {@code endpoints} (in the order, if any, specified
   * by {@code endpoints}), if one is present; otherwise, returns {@code Optional.empty()}.